The distance of the hyades star cluster 14 thus, we obtain the final equation for s (5.2) ty (km/s)-4.74 d (pc) μ ("/year). again, normally this equation will be useful only if the distance is already known since a star's ay is not directly measurable. moving cluster parallax earbhy star clusters provide a unique exception to the above rule of thumb since the space velocities can be found without first knowing the distance. this is possible because accurate proper motions are measurable for these nearby stars, which are all located at approximately the same distance xo (a) a distant view of the cluster motion (b) star motions as seen fron earth figure 5.4 moving cluster if a group of stars moves exactly together, the stars should have parallel space velocity vectors as the cluster moves through space just as two parallel railroad tracks appeiar to converge in the distance, so also will parallel star paths. this point of convergence is determined on a chart of the sky by simply extending the lines of proper motion of each stat, and finding their point of intersection (figure54)